In the prior art, semiconductor read-only memories, or ROM's, may be custom-built to include a number of cells that are conducting with the remainder of the cells non-conducting unless activated by a control gate. The latter cells comprise enhancement devices while the former comprise depletion devices. The particular selection of cells that are formed as depletion cells is unique to the information or program that is to be stored in the memory. The end user of the memory usually specifies which cells are to be depletion type cells and the ROM is then custom built to that specification.
Forming a depletion device requires certain steps early in the manufacturing process in the prior art. Hence the customer must wait for the entire manufacturing process to be completed after he specifies his desired program. If the device could be manufactured nearly to completion, and then programmed according to the customer's specifications, turn-around time from order to delivery would be considerably reduced. The present invention accomplishes this end.